Electromagnets are widely used in modern society in a number of applications. As is well known, in its most basic sense, an electromagnet includes one or more cable leads which are coiled around a central axis. When electricity in the form of current (familiarly referred to as the transport current) is passed through the leads, a magnetic field is generated in the direction of the central axis. With the advent of relatively high temperature superconductors, i.e., superconductors which become electrically superconductive when cooled to temperatures above the boiling point of nitrogen, superconducting cables and, hence, relatively more advanced "superconductor" electromagnets and electrical transmission lines, can be produced.
One problem inherent in a multifilamentary superconducting cable is that energy-wasting eddy currents are induced in the superconducting cable when the cable is subjected to changing transverse magnetic fields. Such changing magnetic fields occur when a superconducting magnet is charged up or discharged, pulsed, or when it is in an alternating field during operation. Similarly, such changing magnetic fields occur when a superconducting transmission line is used to transmit ac current. This generation of eddy currents is particularly nettlesome in the case of electromagnets which use superconducting cables for three reasons. First, the eddy currents in the cables are dissipated in the form of heat, which warms the superconducting cable and thus reduces the current-carrying capacity of the superconducting cable. Indeed, if the heat generated by the eddy current dissipation is great enough, the superconducting cable may be heated to above its critical temperature and accordingly cease to be superconductive altogether. A corollary result of this heat build up is the need for energy dissipation and a consequent time delay in attaining an effective operational level. Second, a superconducting cable at a given temperature can conduct only a finite total amount of current. Consequently, the higher the eddy currents are in the cable, the lower is the amount of useful transport current that can be carried by the cable for generation of the magnetic field. Thirdly, the presence of eddy currents prevents stable operation of a superconducting electromagnet until the eddy currents have dissipated, so it is desirable to have a short time constant for this dissipation to occur.
In light of the above discussion, it is preferable that the generation of eddy currents within the coils of a superconducting cable of an electromagnet be minimized. It is known that one way of reducing induced eddy currents in multifilamentary superconductor cables is to transpose the superconducting filaments which make up the superconducting cable. By the term "transpose", it is meant that the individual filaments which make up the cable periodically change places with each other along the length of the cable. To effect such transposition, the axial path cf each filament extends from a starting coordinate to a coordinate which is radially 180.degree. opposite the starting coordinate, and then continues about the longitudinal axis back to a coordinate which is radially identical to the starting coordinate. The transposed path for each filament in the cable repeats itself as many times as necessary along the length of the cable. Further, it has been found that as the number of such transpositions of each superconducting filament about the longitudinal axis of the cable per unit of length of the cable is increased, unwanted eddy current effects in the superconducting cable is decreased.
Increasing the number of transpositions of the filaments per unit length of cable, however, requires a sharper bending angle in each superconducting filament with respect to the longitudinal axis of the cable. Unfortunately, the superconducting materials used to construct the superconducting filaments are typically ceramic and are thus very brittle. Importantly, while such ceramic filaments can withstand some compression, they cannot tolerate much tension. Specifically, these ceramic filaments easily break when subjected to tensile stresses as may be imposed on the filaments when the wires are bent. Consequently, if the allowable tensile limits of the superconducting filaments are exceeded during bending, the filaments weaken, fracture, and break. This, of course, results in a loss of the wire's superconducting properties. On the other hand, for wires straightened out with little or no transposition per unit of length, unwanted eddy currents are increased.
In light of these limitations, the present invention recognizes that the transposition of superconducting filaments for the reduction of unwanted eddy currents must be balanced by the competing concern for the lack of flexibility and brittleness of the superconducting filaments. The present invention accommodates these competing concerns by an arrangement of superconductor filaments in a superconducting cable which obtains the benefits of wire transposition, while staying within acceptable mechanical stress limits for the filaments.
Accordingly, it is an object of the present invention to provide a multifilamentary superconducting cable which uses transposition to effectively reduce unwanted eddy current losses. It is another object of the present invention to provide a multifilamentary superconducting cable which has sufficient flexibility so that it can be formed into desired shapes without causing loss of superconductivity. Still a further object of the present invention is to provide a multifilamentary superconducting cable which is durable and reliable in operation. Yet another object of the present invention is to provide a multifilamentary superconducting cable which exhibits desirable performance, yet is cost-effective in its manufacture.